Process for preparing cellulose pulp by alkaline digestion while inhibiting extraction of hemicellulose

ABSTRACT

A process is provided for preparing cellulose pulp by the alkaline digestion of lignocellulosic material while inhibiting extraction of hemicellulose by employing an alkaline cooking liquor having a high concentration of substantially nondegraded hemicellulose having a degree of polymerization of at least 80. The cellulose pulp product thus contains within its fibers much or substantially all of the hemicellulose present in the natural lignocellulosic material.

United States Patent Inventors lngemar Liss-Albin Croon Ornslroldsvik;Sten Dillen, Alfredshem; Sture Noreus, Sundasen, all of Sweden Appl. No.28,790 Filed Apr. 15, 1970 Patented Nov. 2, 1971 Assignee Mo och DomsjoAktiebolag Ornskoldsvik, Sweden Priority Mar. 3, 1966 Sweden 2,823/66Continuation-impart of application Ser. No. 618,360, Feb. 24, 1967, nowabandoned.

PROCESS FOR PREPARING CELLULOSE PULP BY ALKALINE DIGESTION WHILEINHIBITING EXTRACTION 0F HEMICELLULOSE 16 Claims, 3 Drawing Figs.

Primary Examinerl-loward R. Caine Attorney-James & Chapman ABSTRACT: Aprocess is provided for preparing cellulose pulp by the alkalinedigestion of lignocellulosic material while inhibiting extraction ofhemicellulose by employing an alkaline cooking liquor having a highconcentration of substantially nondegraded hemicellulose having a degreeof polymerization of at least 80. The cellulose pulp product thuscontains within its fibers much or substantially all of thehemicellulose present in the natural lignocellulosic material.

PATENTEUHM l9?! 3.'s17;431

SHEET 1 BF 3 Fig.8

2 woodw whiie I -stobilizer sieo hemicelluloserich 518cm alkalinecooking liquor 11 black n pulp producf retaining hemiceilulose PROCESSFOR PREPARING CELLULOSE PULP BY ALKALINE DIGESTION WHILE INHIBITINGEXTRACTION F HEMICELLULOSE This application is a continuation-in-part ofSer. No. 618,360, filed Feb. 24, 1967 and now abandoned.

The present invention relates to a process for the manufacture ofcellulose pulp by alkaline cooking or digestion of lignocellulosicmaterial, such as wood, while inhibiting the extraction or dissolutionof hemicellulose from the lignocellulosic material during the alkalinecooking process to thereby produce a pulp containing within the pulpfibers more of the hemicellulose originally present in thelignocellulosic material than has been retained previously.

FIG. I is a sectional view of a digestion apparatus which can beemployed in accordance with the instant invention.

FIGS. 2 and 3 plot lignin-free yield against kappa number.

The term hemicellulose is used in the usual sense to include allpolysaccharides naturally present in lignocellulosic material which canbe dissolved in aqueous solutions of alkali. Hemicellulose mainlycomprises hexosans, such as glucomannans, pentosans, such as xylan,which may contain uronic acids, and nonglucose polysaccharides such asarabinogalactans, and also includes a number of cellulose degradationproducts. The total percentage of hemicellulose in different types oflignocellulosic material varies, and can be within the range, forexample, from about I to about 2 percent in soft wood, and from about 22to about 35 percent in hard wood.

In the manufacture of cellulose pulp by the alkaline digestion of alignocellulosic material, such as wood the wood is progressively heatedor cooked in an alkaline cooking liquor at a predetermined temperatureuntil it is delignified to the desired degree. The alkaline cookingliquor employed varies depending on the type of process employed. Forexample, in the kraft process, the alkaline liquor comprises a mixtureof white liquor and black liquor and includes an alkaline material, suchas sodium hydroxide, and neutral or alkali metal salts, such as sodiumsulfate, sodium carbonate, sodium sulfide, sodium sulfite, sodiumhydrosulfide and sodium thiosulfate and water. The black liquor ismerely white liquor which has been employed in an alkaline digestionprocess and thus contains dissolved hemicellulose and other organicmaterials extracted from the lignocellulosic material. The coolingliquor employed in the soda process is based on a mixture of largeamounts of sodium hydroxide and relatively small amounts of sodiumcarbonate, and includes some or all of the salts employed in the cookingliquor of the kraft process. The sodium hydroxide and the salts in thecooling liquor function to open the alkylaryl ether bonds of lignin inthe lignocellulosic material, to produce lignin degradation productswhich are soluble in the alkaline medium. In both the kraft and sodaprocesses, wood is normally cooked at a temperature within the rangefrom about 50 to about 190 C. for varying periods of time, dependingupon the type of wood being cooked, and the properties desired in thepulp product.

Lignocellulosic material contains relatively large quantities ofhemicellulose. During the alkaline cooking of the lignocellulosicmaterial, a large portion of the hemicellulose, up to percent, based onthe dry wood weight, is extracted into the cooking liquor, and anadditional 10 to percent may be extracted and degraded. The percentageof hemicellulose extracted from the lignocellulosic material anddissolved in the cooking liquor increases to a maximum during the firstportion of the cooking process, while the temperature is being raised tothe maximum level, and then falls, due to degradation of thehemicellulose into soluble products. However, very little, if any, ofthe hemicellulose is precipitated onto the cellulose, and thus asubstantial amount of the hemicellulose is lost.

The presence of hemicellulose in cellulose pulp is often desired. Highhemicellulose paper pulp increases in strength more quickly on beatingthan does pulp containing little or no hemicellulose. Furthermore, papermanufactured from highhemicellulose pulp generally has a greaterbreaking length (tensile strength), although it may have a somewhatreduced tearing strength. Thus, in many instances, it would be desirableto inhibit the dissolution of hemicellulose from the lignocellulosicmaterial being cooked in alkaline cooking liquor, so as to ensure thatas much as possible of the hemicellulose naturally present in the woodremains in the cooked pulp.

To restore hemicellulose lost by extraction, attempts have been made tocombine with the cellulose pulp the hemicellulose extracted frompreviously cooked lignocellulosic material, and dissolved in the cookingliquor. For example, a cooking liquor high in hemicellulose has beenremoved during the cook, and returned to the cooking liquor at the finalstage of the cook, in order to precipitate hemicellulose onto the pulpfibers. However, this procedure has not been too successful, becauseactive alkali is also withdrawn from the cooking process with thehemicellulose, and returned to the final stage of the cooking processand thus inhibits the precipitation of the hemicellulose. In order topromote precipitation of the hemicellulose from the cooking liquor ontothe pulp fibers, acid has been added to the cooking liquor rich inhemicellulose. However, this results in an undesirable consumption ofacid and alkali, which could otherwise have been used in the digestionprocess. In both cases, hemicellulose precipitated onto the fibers ofthe pulp product is not returned to within the fibers in the conditionin which it was originally present in the lignocellulosic material, butlies on the surface. Here, it is easily lost in subsequent processing.Thus, for example, when the pulp is submitted to bleaching operations,much of the hemicellulose precipitated on the pulp fibers is removed.

ln normal sulfate pulping, a portion of black liquor (liquor drawn offat the end of the cooking process) is recycled for use in the nextcooking process. The purpose of this step is to increase the solidscontent of the cooking liquor containing the substantially cooked wood.The recycled black liquor contains of course, a certain amount ofhemicellulose. However, the hemicellulose dissolved in the black liquoris substantially degraded during the latter portion of the cookingperiod, i.e. the time during which the cook is held at maximumtemperature (normally to C.) and has no appreciable influence on thepercentage of hemicellulose in the pulp produced in the next cooking.

US. Pat. No. 1,737,590 to .lohnsen describes a cooking process using anacid or neutral cooking liquor to which soluble carbohydrates are added,such as hexoses, pentoses and hexosans and pentosans. These are thehydrolysis products of the carbohydrates present in the wood, and can beadded as waste cooking liquor from a previous cooking operation.However, the hydrolyzed or degraded carbohydrates are not effective ininhibiting solution of the nonhydrolyzed hemicellulose of the wood.Moreover, .lohnsen removes the lignin content from the cooking liquorbefore recycling it.

The present invention provides a process for preparing cellulose pulp bythe alkaline digestion of lignocellulosic material, retaining in thepulp as much as possible of the hemicellulose originally present in thelignocellulosic material. Whereas in the normal process, a hemicelluloseretention of 40 percent or even lower is normal, in the process of theinvention, hemicellulose retention of about 50 percent or more isobtainable. The hemicellulose that is retained is not extracted at all,and so it remains in its original condition in the fibers, and thus anincrease in yield of pulp is obtained and hemicellulose in the pulp isnot lost during subsequent treatment of the pulp, e.g., on bleaching.

In accordance with the invention, the hemicellulose originally presentin the lignocellulosic material to be cooked is substantially preventedfrom being extracted from the lignocellulosic material and from beingdissolved in the alkaline cooking liquor during the cook, by providing ahigh concentration of substantially nondegraded hemicellulose in thealkaline cooking liquor at the beginning of the digestion. Preferably,the alkaline cooking liquor has as high a concentration as possible ofsubstantially nondegraded hemicellulose, and preferably at least Ipercent by weight of the cooking liquor.

Thus, in accordance with the process of the instant invention forpreparing cellulose pulp by alkaline digestion of lignocellulosicmaterial, lignocellulosic material is heated in an alkaline cookingliquor containing dissolved substantially nondegraded hemicellulose, ata digestion temperature below that at which dissolved hemicellulose isdegraded or decomposed and for a time to convert the lignocellulosicmaterial to pulp. After cooking, the alkaline liquor can be withdrawnand the alkali concentration restored, after which the-alkaline liquorcan be used for a further cooking operation, due to the fact that thedissolved hemicellulose is not materially degraded or decomposed.

The degree of degradation or decomposition of hemicellulose is measured,in accordance with the invention, in terms of the degree ofpolymerization of the hemicellulose. 1f the degree of polymerization islow, the degree of degradation is high. A hemicellulose that issubstantially not degraded nor decomposed, in accordance with theinvention, hasa degree of polymerization, abbreviated fin, of at least80, preferably at least 100, and most preferably at least 120.

The determination of fin is according to the procedure described at page697 of the article by S. Axelsson et a1., Dissolution of HemicelluloseDuring Sulfate Pulping, Svensk Papperstidning 1962 693 No. 18 (Sept. 30,1962). This procedure is as follows:

The cooking liquor (400 ml.) is neutralized with acetic acid and pouredinto alcohol 1600 ml.). The precipitate is isolated by centrifugation,washed three times with alcohol: water (401), three times with alcohol,three times with ether and dried in vacuo.

The fraction (2 g.) is delignified with ml. chlorine dioxide solution(20 g. ClO,/l) and the hemicellulose recovered by precipitation withalcohol. The [1 is determined viscometrically, using a Wagner-Russelviscosimeter. Cupriethylene solution (0.25 M) is used as solvent. Thedilutiortio different concentrations is made in the viscosimeter. TheDPn is calculated from fin =K-[n]. The constant is given the value 212according to Glaudemans, and Timell, Svensk Papperstidning 6i (1958) 1.

An alkaline cooking liquor having a high concentration of substantiallynondegraded hemicellulose can be prepared by heating an alkaline cookingliquor and a lignocellulosic material to the maximum temperature atwhich the cook is to be carried out. This temperature is within therange from about 25 C. to about 190 C., preferably 120 to 150 C. At thistemperature, it is assured that the concentration of hemicelluloseextracted from the lignocellulosic material in the alkaline cookingliquor will be at a maximum, and this will inhibit further dissolutionfrom the lignocellulosic material to be treated therewith. Such analkaline cooking liquor usually has a concentration of at least 1percent up to about 10 percent hemicellulose. The liquor is withdrawnwhen ready, cooled, and mixed with lignocellulosic material for the cookin accordance with the invention. The liquor can be reused for a numberof cooks, if the alkali concentration is replenished.

The instant process can in this way be carried out continuously by usinga long-dwell digester tower, and recirculating the alkaline cookingliquor. The alkaline cooking liquor having a high concentration ofhemicellulose is withdrawn from the digester at a temperature below thatat which hemicellulose is degraded,,within the range from about 100 C.to about l70 C., according to the process used, cooled preferably tobelow 120 C., recycled to the beginning of the digester for blendingwith fresh lignocellulosic material, and the alkali concentrationreplenished by addition of the necessary amount of fresh cooking liquor,i.e. white liquor. The withdrawal of the cooling liquor is performed insuch a manner and in such an amount that the amountvof alkali retainedin the pulp is sufficient to complete the delignification ofthe pulp.

The yield obtained in the process of the instant invention can beadvantageously improved by admixing in the alkaline cooking liquor acompound capable of improving the resistance of the hemicellulose in thelignocellulosic material and in the cooking liquor to degradation at thecooking temperatures. The stabilizer is added to the hemicellulosecontaining liquor in an amount within the range from about 0.1 to about3 percent by weight of the lignocellulosic material. Examples ofstabilizers which inhibit degradation of hemicellulose and which aresuitable for use herein include alkali metal and ammonium polysulfides,such as sodium polysulfide, potassium polysulflde, lithiumv polysulfideand caesium polysulfide; alkali metal borohydrides, such as sodiumborohydride, potassium borohydride, lithium borohydride, rubidiumborohydride, and caesium borohydride; hydrazine, and hydrazinederivatives. Sodium polysuliides are the preferred stabilizers.

Degradation of the hemicellulose can also be inhibited by keeping thetemperature of the hemicellulose-containing alkaline cooking liquorbelow about 120 C. as much as possible.

The process of the instant invention is applicable to all types ofalkaline cellulose pulping processes, including the sulfate and sodaprocesses, as batchwise and continuous operations.

In the case of both batch and continuous alkaline cooking processes, theratio of alkaline cooking liquor: lignocellulosic material is within therange from about 3:1 to about 45:1. The mixture of liquor andlignocellulosic material is progressively heated from the startingtemperature of from 50 to C. to the maximum cooking temperature, withinthe range of from about 150 to about 190 C., over a cooking time withinthe range of from about three-fourths to about 4 hours. The maximumtemperature is maintained for a time within the range of from aboutone-half to about 5 hours.

Asthe concentration of hemicellulose that can be reached in an alkalinecooking liquor is a function of alkali concentration, digestiontemperature and time, the concentration of hemicellulose at the startshould be as close as possible to that reached in a digestion underidentical or comparable temperatures and times. Of course, since therecycled liquor is usually diluted with water and white liquor, it isnot quite possible to reach this concentration. Usually, the amount ofhemicellulose present in the liquor is within the range from about 1percent to about 10 percent by weight of the liquor. The alkalinecookingv liquor should preferably be removed from the lignocellulosicmaterial when the maximum digestion temperature has been reached, orbefore this. Care should be taken so that the liquor is drawn off beforea temperature is reached at which the hemicellulose is degraded to anundesirable degree.

The digestion temperature and time and alkali concentration depend onthe kind of wood pulped. For example, where pine wood is to be cooked,the cooking liquor is drawn off at a temperature of at least C. andsuitably within the range from about to about 170 C. and preferably fromabout 160 to about 170 C., and in the case of birch at a temperature ofat least 120 C. and suitably within the range of from about 130 to about170 C., and preferably from about to about C. As much liquor as possibleshould be drawn oh so as to maintain the supply of recirculating cookingliquor, and prevent contamination of the pulp with cooking liquordegradation products during the remainder of the digestion, which takesplace at maximum temperature. There must be a sufficient amount ofalkali remaining to complete the diligniftcation. This as a rule meansthat enough liquor should be withdrawn so that the ratio of liquorremaining to wood to be digested is within the range from about 1:1 toabout 2.5:1 and preferably about 2:1. The digestion is then terminatedin vapor phase, by cooking at or about the max- FIG. 1 is a sectionalview of a digestion apparatus which can be employed in carrying out acontinuous alkaline cooking process in accordance with the instantinvention. The apparatus comprises an elongated digester vessel 1containing conduits or inlets 2 and 4 for wood and white liquor, respectively, and outlet 4 for the cellulose pulp product. Steam heaters 5 and6 are provided for heating liquor circulating from the digester vessel 1via pipes and 16, 17 and 18, respectively. The digester l is equippedwith an alkaline liquor recycle line, composed of withdrawal line 7,cooler 8, mixer 9, and recycle feed conduit 10 for recycling thehemicellulosecontaining cooking liquor. Digester 1 is also provided withoutlet 11 for withdrawal of the residual cooking liquor (black liquor)after cooking is completed, separation recirculation conduit 13 and washliquor introduction conduit 12 for washing the pulp prior to dischargingit from the digester 1 via line 4, and heater 14 for heating the washliquor.

In carrying out the process of the instant invention employing theapparatus of FIG. 1, a lignocellulosic material, e.g. wood, alkalinecooking liquor containing a high concentration of substantiallynondegraded hemicellulose (recycled via lines 7 and 10) and white liquorfor alkali replenishment of the alkaline liquor, are introduced throughconduits 2, 10 and 3, respectively, into the top of the elongateddigester 1. As the mixture travels downwardly through the digester toline 7, the mixture is gradually brought to a temperature of from 110 to160 C. by circulation through heater 5. The alkaline cooking liquor isdrawn off through the conduit 7, cooled to 50 to 120 C. in cooler 8,mixed with polysulfide stabilizer in mixer 9, and recycled to the top ofthe digester 1 via conduit 10. The remaining liquor and wood contents ofthe digester are gradually brought to a temperature of from 150 to 190C., as they progress downwardly from line 7, by circulating throughheater 6, to complete the cooking, and move downwardly through thedigester to line 11. Black liquor is removed through the conduit 11,which is at the completion of the digestion. The digested pulp iscountercurrently washed with washing liquor introduced through conduit12, recirculated and heated in heater 14 via lines and 21. The washedpulp is finally removed from the digester via line 4.

The rates of withdrawal of alkaline recycled liquor, black liquor, andwood pulp and the rate of introduction of wash liquor are adjustedaccording to the digestion conditions desired, and will vary with thetype of wood and digestion time and temperature, and the alkaliconcentrations.

The lignocellulosic material which can be converted to cellulose pulp inaccordance with the instant invention includes wood, in any form such aswood chips, shavings, sawdust, splinters, etc. and includes woodcontaining alkylaryl ether lignin bonds, such as spruce, pine, redwood,larch, Douglas fir, h :mlock, and wood from deciduous trees such asbirch, oak, aspen, beech, eucalyptus, and other lignocellulosicmaterials such as straw, bagasse, bamboo, and rushes.

The process of the instant invention gives an increased pulp yield overthat obtained in conventional alkaline digestion processes. For example,when pine wood is employed as the lignocellulosic material, theinvention provides an increase in yield of pulp product of approximately0.3 percent (calculated on the weight of wood cooked) without using astabilizer, and an increase in yield of up to about 3.5 percent when astabilizer is used. When birch wood is cooked, the instant processprovided an increase in yield of pulp product of about 0.6 percentwithout a stabilizer and 3.5 percent with a stabilizer. In the case of apulp plant manufacturing 100,000 tons of pulp each year, an increase inyield of 1 percent (based on the weight of wood cooked) meansapproximately 2,000 tons of pulp for birchwood and approximately 2,200tons of pulp for pinewood.

The following examples in the opinion of the inventors representpreferred embodiments of their invention.

. tions, percent...

EXAMPLE 1 Air-dry birch chips, free of visible knots and bark, weresubmitted to alkaline digestion in accordance with the process of theinstant invention (Runs 5 to 7) and in accordance with prior artprocesses (Runs 3, 4 and 8) and with other comparable processes (Runs 1and 2). The digester employed in all runs was a 2-liter autoclave in anelectrically heated glycol bath. The charge of birch chips and alkalinecooking liquor (having the composition set out below), was introducedinto the digester and heated up. When a temperature of 150 C. had beenreached in the digester, an amount of alkaline cooking liquorcorresponding to somewhat more than half the amount of alkaline liquorin the digester was drawn off. The withdrawn liquor was cooled, andanalyzed with respect to active alkali and sulfide. Certain portionswere stabilized with a polysulfide in the following manner: an amount ofliquor which corresponded to about half the liquor: wood ratio at thebeginning of the digestion process was heated to C., admixed withpolysulfide and then allowed to stand at 100 C. for 15 min.

The recycled liquor (stabilized or unstabilized) containinghemicellulose was mixed with white liquor and birch chips in thedigester, and the proportions of white and recycled liquors wereadjusted so as to give substantially the same percentage of alkali inthe cooking liquor in each cycle. Liquor was continuously drawn off fromthis cook in a similar manner at 150 C., stabilized in the same way asthe liquor previously drawn off, and then returned to the digester withmore birch wood and white liquor. A total of seven cooks were run usingrecycled alkaline cooking liquor and fresh white liquor in this way,employing the following process conditions:

Process Conditions:

Wood-to-liquor ratio at the be- 0.25 kg. calculated on bone dry giriningof the cooking process. birch wood per liter of liquor. Initial cookingtemperatures. Start80 C. Maximumminutes to heat up to 150 0. Liquordrawn off to a wood-to- 150 C. for 15 minutes.

liquor ratio of 1:2.

Final cook temperature 150-l58 C. for 15 minutes; 158 C.

[or minutes. Chemical charges:

Alkali charge 17 and 20% effective alkali (Na0H+ Na,S calculated atNaOH), including the alkali in the returned cooking liquor.

Sodium polysulfide for stabiliz- 0%, 1% and 2% sodium polyinghemicellulose. sulfige-suliur calculated on the woo Bleaching BleachingNaOH NaOCl C10 conditions C1 NaOH Pulp concentra- Time in hoursTemperature, 0.

Change of active chlorine, ercent ofp p 1 Multiplied by Kappa. number.

The following procedures were used in Runs 5,6 and 7, in accordance withthe invention: Run 5 The alkaline cooking liquor containinghemicellulose was V TABLE II withdrawn at 150 C., and recycled withwhite liquor. Bleach.

Run pp (Curve 2) No. Method number yield Run 6 The same as Run 5, but 1percent sodium polysulfide was 5 1 withdrawal liquor, water fig addedbefore recycling the cooking liquor. 2 u withdrawal liquor, black liquoradded fig 32, 7; (Curve 6, FIG. 2) 0 No withdrawal of liquor, wateradded-.." Run 7 15 8 95 6 No withdrawal of liquor, black liquor The sameas Run 6, but 2 percent SOCllUm polysulfide as added 5 Withdrawal ofliquor, recycling liquor 0% added. polysulfide 25. 1 93. 5 In additionto the above, the following runs using cooking 6 wmll gvg of liquor,recycling liquor 1% 8;:

po ysu e gggitialsxses were c?rr1ed out for comparison with the above 7Withdrawal of liquor, recycling liquor 2% gig 8125.: g process 0 theinvention. polysulfide 16 3 1 Run 1 8 sorpiion l i ii H so dd d 17 s 95a At the beginning of the cooking process, fresh white liquor w gggiggg3, 83%: af 3 and water was added, but no hemicellulose-containing Nowithdrawal, H 801 added 18.7 94.9 liquor. Half the liquor present wasdrawn off at 150 C.

(Curve 1, FIG. 2) Run 2 At the beginning of the cooking process freshwhite liquor 20 and black liquor were added, as is normally done whendigesting sulfate pulp on a factory scale. Half the liquor Thelignin-free yield of pulp obtained for the eight runs present was drawnoffat 150 C. made is plotted in FIG. 2 against kappa number (which canbe (Curve 2, FIG. 2) converted into the percent lignin present in thepulp product Run 3 by multiplying by 0.15). The comparison between thevarious Cooking as in Run 1 without drawing offliquor at 150 C. cookingmethods was made at kappa no. 18.. This kappa (Curve 3, FIG. 2) numberis considered normal in factory cooking of birch. Run4 Curve 1 showsthat Run 1 using fresh white liquor as the Cooking as in Run 2, but noliquor was drawn off at 150 C. cooking liquor, gave the lowest yield.Hemicellulose was dis- (Curve 4, FIG. 2) solved in the liquor duringcooking, and was drawn from the Run 8 charge; the yield is thus low.

Cooking as in Run 2, including drawing offliquor rich in he- A cook likeRun I where no liquor is drawn off, so a micellulose at 150 C. butreturning this liquor to the final minimum of hemicellulose is lost inthe liquor, is represented stage of the digestion. (sorption digestion).by Curve 3. The yield in this case lies approximately 0.9 per- (Curve 8,FIG. 2) cent higher than in curve 1.

Average values of the yields in the runs according to the in- Curves 2and 4 show the yield in the case when black liquor vention (Runs 5, 6and 7) as compared with the other methods containing degradedhemicellulose, removed from the final (Runs 1, 2, 3, 4, and 8) are shownin the following table. Ligstage of the cooking process, was usedinstead of fresh water nin-free yield refers to the percent pulp productbased on in preparing the cooking liquor, with withdrawal of liquorweight of llgnocellulosic material cooked. (Run 2) and withoutwithdrawal (Run 4).

TABLE I Total mean Mean Active alkali, percent of wood lignin-ireeKapga- Run No. Method yiel num er Charged Charged+recycled 1 d wa o qwater added. no recycling 3 5': 5 2&2 :jjjjjjjjjjjjjjj I: 2 Withdrawalof liquor, black liquor added, no recycling 25:; i3 gg 1- 3 Nowithdrawal of liquor, water added, n0 recycling 2i; fjg Q22 3:1" I: I"I: 4 N with 52.1 17.5 19.8 "i0. 2 o drawal of liquor, black liquoradded, no recycling" 51' 75 m 3 23 3 7 6 Withdrawal, recycling liquor(0% polysulfid 231% 3:? 5% 32:2 2 Withdrawal, recycling liquor (1%polysulfide) @2 5 f2; 3 l 2: 7 Withdrawal, recycling liquor (2%polysulfide) g1? fig 1 ,312 5, ,3 51. 3 19.0 1 23. 6 2 23. 6 3 smmndigest! 1 In the case of cooking where polysuifide was added at thebeginning of the process; requires an extra alkali charge of 0.4% percharged percent polysulfide sulfur (on low sulfidities). If thesulfidity is high, a higher alkali charge is necessary.

2 With H1804 additive in the final stage of the cooking process.

As can be seen from table I, the cooking processes carried out accordingto the invention i.e., Runs Nos. 5, 6 and 7, have a considerablyincreased yield as compared with remaining cooks.

in table II below are shown the bleaching yields for these cooking runs.The bleach yield drops with high kappa numbers, owing to the fact thatthe pulp then contains more lignin, which is bleached away. Thebleaching yields are highest for cooking processes inwhich liquor isreturned according to the invention (Runs Nos. 5, 6 and 7) which showsthat the thusproduced pulp withstands bleaching equally as well asconventionally digested pulp.

Substantially higher yields were obtained when in accordance with theprocess of the instant invention, Runs 5 to 7, an alkaline liquor highin-hemicellulose was withdrawn from the digester, replenished with whiteliquor, and recycled to the start of the digestion. Run 5 (Curve 5) havea yield a little more than 0.6 percent above Run 3. When the recycledliquor (Curves 6 and 7) was stabilized with polysulfide, a 2.5 percenthigher yield was obtained than in Run 3. A normal polysulfide cook givesapproximately 1 percent yield-gain per added percent polysulfide sulfur.

It can be seen from Curve 8 that the increase in yield was very smallwhen withdrawn liquor rich in hemicellulose was returned to the finalstage of the cooking process and in fact even lower than in the case ofa conventional cook where no liquor was drawn off (Runs 3 and 4).

The degree of polymerization (l Pn) of the hemicellulose recycled inRuns 5 to 7 was determined and found to be as follows:

Run No. fin 5 149 6 151 7 152 This shows that the hemicellulose was notdegraded to any significant degree.

EXAMPLE 2 The procedure of Runs l, 2,4, 5, 6 and 7, and example 1 wasrepeated except that pinewood was employed instead of birchwood. Thecook was heated to the temperatures set out below, liquor being drawnoff at 170 C. The pulps were not bleached.

Recycling cooking liquor rich in hemicellulose Wood-to-liquid ratio 1:4Initial Cook Teln- Start- 80 C. after perature 10 min.

Maximum-180 min. to heat up to 170 C. with drawal of liquor after 10min.

Final Cook Tem- 170 C. for 120 min.

perature TABLE III Total lignin- Run No. Method free yield 1 withdrawalof liquor,

water added 45.6 1 withdrawal of liquor,

black liquor added 45.8 4 A no withdrawal of liquor,

black liquor added 45.9 5 withdrawal of liquor, 7

recycling of liquor 0% polysulfide 46.2 6 withdrawal of liquor,

recycling of liquor 1% polysulfidc 47.4 7 withdrawal of liquor,

recycling of liquor 2% polysulfide Kappa number 32 cooking according tothe invention (Curves 5, 6 and 7) was considerably higher than in thecase of other cooking processes (Runs 2,4) used as a comparison.Stabilization with 1 percent sodium polysulfide (Run 6, Curve 6) gave a1.5 percent yield gain as compared with Curve 4, (without drawing offliquor). A further addition of 1 percent sodium polysulfide (Run 7,Curve 7) merely gave a 1 percent gain, a gain which is not more thanthat obtained in a normal polysulfide cooking process.

The degree of polymerization of the hemicellulose recycled in Runs 5 to7 was determined, and found to be as follows:

Run fin 5 103 6 I04 7 I05 This shows that the hemicellulose was notdegraded.

substantially EXAMPLE 3 Birch was cooked as in example 1. However, thecooking liquor was heated to 158 C. which took about 120 minutes,maintained at 158 C. for 15 minutes and then drawn off at 158 C. Onlycooking processes which involved drawing off liquor and restoring saidcooking liquor (with and without stabilization) were carried out (Runs1, 3, 5, 6, 7).

Mean values for the yields in various cooking methods are given in table1V below.

As can be seen from the table, the cooking processes carried outaccording to the invention (Runs 5, 6 and 7) increased the yield ascompared with Runs 1 and 3.

The degree of polymerization of the hemicellulose recycled in Runs 5 to7 was determined and found to be as follows:

Run No. DTn 5 I29 6 130 7 130 This shows the hemicellulose was notsubstantially degraded.

Having regard to the foregoing disclosure, the following is claimed asthe inventive and patentable embodiments thereof:

1. A process for digesting lignocellulosic materials while retaininghemicellulose within the resulting cellulose pulp material, whichcomprises heating a lignocellulosic material in an alkaline cookingliquor containing dissolved substantially nondegraded hemicellulose in aconcentration of at least 1 percent and having a degree ofpolymerization of at least at a digestion temperature below that atwhich dissolved hemicellulose is degraded and for a time to convert thelignocellulosic material to pulp, and recovering a pulp productretaining a substantial proportion of the hemicellulose therewithin.

2. A process as in claim 1 wherein the alkaline cooking liquor containsat least about 2 percent by weight of substantially nondegradedhemicellulose.

3. A process as in claim 1 wherein the alkaline cooking liquorcontaining dissolved hemicellulose is prepared by heating an alkalinecooking liquor and a lignocellulosic material to a digestion temperaturewithin the range from about to 1 about 190 C. to extract hemicellulosefrom the lignocellulosic material and dissolve the hemicellulose in thealkaline cooking liquor.

4. A process as in claim 3, wherein the alkaline cooking liquor iscooled to a temperature below about 120 C. before it is employed incooking lignocellulosic material.

5. A process as in claim 3 wherein a stabilizer which inhibitsdegradation to hemicellulose to a degree of polymerization below 80 isadmixedwith the alkaline cooking liquor containing dissolvedhemicellulose prior to employing the cooking liquor to cooklignocellulosic material.

6. A process as in claim 5 wherein the stabilizer is added to thecooking liquor at a temperature below about 120 C.

7. A process as in claim 5 wherein the stabilizer is selected from thegroup consisting of alkali metal and ammonium polysulfides, alkali metalborohydrides and hydrazine.

8. A process as in claim 5 wherein from about 0.1 to about 3 percentstabilizer based on weight of lignocellulosic material is added to thealkaline cooking liquor.

9. A process as in claim 1 wherein the alkaline cooking liquorcontaining dissolved substantially nondegraded hemicellulose is recycledfrom a previous lignocellulosic material digestion carried out underconditions to maintain a degree of polymerization of the dissolvedhemicellulose of at least 80.

10. A process as in claim 1 for digesting lignocellulosic materialswhile retaining hemicellulose within the resulting cellulose pulpmaterial, which comprises heating a lignocellulosic material in analkaline cooking liquor at a digestion temperature within the range fromabout 100 to about 190 C. to extract substantially nondegradedhemicellulose from the lignocellulosic material and dissolve thehemicellulose in the alkaline cooking liquor, so that the liquorcontains at least about 1 percent hemicellulose having a degree ofpolymerization of at least 80, withdrawing at least a portion of thealkaline cooking liquor containing such dissolved substantiallynondegraded hemicellulose, cooling the withdrawn liquor to a temperaturebelow about l20 C., recycling the cooled liquor, with addition of alkaliif necessary, and digesting a fresh portion of lignocellulosic materialin accordance with the digestion process of claim 1 in the presence ofsuch substantially nondegraded hemicellulose.

11. A process as in claim 10 wherein the amount of cooking liquorremaining after a portion of the cooking liquor is drawn off, issufficient for completing the digestion of the partially cookedlignocellulosic material.

12. A process as in claim 1 1 wherein the amount of cooking liquorremaining is at least 1.5 liters per kg. lignocellulosic material.

13. A process as in claim 10 wherein the lignocellulosic material isbirch wood and the alkaline cooking liquor containing dissolvedhemicellulose is drawn off at a temperature within the range from aboutto about l 70 C.

14. A process as in claim 10 wherein the lignocellulosic material ispinewood and the alkaline cooking liquor containing dissolvedhemicellulose is drawn off at a temperature within the range from about120 to about C.

15. A process as in claim 10 wherein the digestion process is carriedout continuously.

16. A process as in claim 10 wherein the digestion process is carriedout batchwise.

i i i i Patent No.

Dated November 2, 1971 lnve Ingemar Liss-Albin Croon et al.

It is certified that error appears in the above-identified patent andthat: said Letters Patent are hereby corrected as shown below:

Column 1, line 26 Column 1 line 43 v Column 1, line 48 Column 4, line 64Column 6, line 58 Column 8, Table I column headed "Mean Kappa Number",line 1 line 2 line 4 line v5 line 6 line 16 Column 8, line 68"dilignification" should be delignification "NaocL" should be NaOCl "2"should be 27 "1.3" should be 17.3

"LO" should be 16.0

"2.0" should be "1.0" should be 16.0

"16.9" should be 16.0

"have" should be gave SZQLWP L 10, .JZLPLL L B J J GLICIGd "Tota'l lignin-free yield" 'Run No. 7, line 46 Insert 55.4

b O v (SEAL) Attest:

,ROBERT GOTTSCHALK EDWARD M. FLETCHER,JR.

Commissioner'of Patents Attesting Officer

2. A process as in claim 1 wherein the alkaline cooking liquor containsat least about 2 percent by weight of substantially nondegradedhemicellulose.
 3. A process as in claim 1 wherein the alkaline cookingliquor containing dissolved hemicellulose is prepared by heating analkaline cooking liquor and a lignocellulosic material to a digestiontemperature within the range from about 100 to about 190* C. to extracthemicellulose from the lignocellulosic material and dissolve thehemicellulose in the alkaline cooking liquor.
 4. A process as in claim3, wherein the alkaline cooking liquor is cooled to a temperature belowabout 120* C. before it is employed in cooking lignocellulosic material.5. A process as in claim 3 wherein a stabilizer which inhibitsdegradation to hemicellulose to a degree of polymerization below 80 isadmixed with the alkaline cooking liquor containing dissolvedhemicellulose prior to employing the cooking liquor to cooklignocellulosic material.
 6. A process as in claim 5 wherein thestabilizer is added to the cooking liquor at a temperature below about120* C.
 7. A process as in claim 5 wherein the stabilizer is selectedfrom the group consisting of alkali metal and ammonium polysulfides,alkali metal borohydrides and hydrazine.
 8. A process as in claim 5wherein from about 0.1 to about 3 percent stabilizer based on weight oflignocellulosic material is added to the alkaline cooking liquor.
 9. Aprocess as in claim 1 wherein the alkaline cooking liquor containingdissolved substantially nondegraded hemicellulose is recycled from aprevious lignocellulosic material digestion carried out under conditionsto maintain a degree of polymerization of the dissolved hemicellulose ofat least
 80. 10. A process as in claim 1 for digesting lignocellulosicmaterials while retaining hemicellulose within the resulting cellulosepulp material, which comprises heating a lignocellulosic material in analkaline cooking liquor at a digestion temperature within the range fromabout 100* to about 190* C. to extract substantially nondegradedhemicellulose from the lignocellulosic material and dissolve thehemicellulose in the alkaline cooking liquor, so that the liquorcontains at least about 1 percent hemicellulose having a degree ofpolymerization of at least 80, withdrawing at least a portion of thealkaline cooking liquor containing such dissolved substantiallynondegraded hemicellulose, cooling the withdrawn liquor to a temperaturebelow about 120* C., recycling the cooled liquor, with addition ofalkali if necessary, and digesting a fresh portion of lignocellulosicmaterial in accordance with the digestion process of claim 1 in thepresence of such substantially nondegraded hemicellulose.
 11. A processas in claim 10 wherein the amount of cooking liquor remaining after aportion of the cooking liquor is drawn off, is sufficient for completingthe digestion of the partially cooked lignocellulosic material.
 12. Aprocess as in claim 11 wherein the amount of cooking liquor remaining isat least 1.5 liters per kg. lignocellulosic material.
 13. A process asin claim 10 wherein the lignocellulosic material is birch wood and thealkaline cooking liquor containing dissolved hemicellulose is drawn offat a temperature within the range from about 130* to about 170* C.
 14. Aprocess as in claim 10 wherein the lignocellulosic material is pinewoodand the alkaline cooking liquor containing dissolved hemicellulose isdrawn off at a temperature within the range from about 120* to about170* C.
 15. A process as in claim 10 wherein the digestion process iscarried out continuously.
 16. A process as in claim 10 wherein thedigestion process is carried out batchwise.